This invention relates to a process for the modification of unsaturated polyolefin rubbers by grafting with other polymers lending themselves to the metathesis reaction, in the liquid phase, preferably in solution, in the presence of a Ziegler-Natta catalyst system comprising (a) a soluble compound of a metal of Subgroups V through VII of the periodic table, and (b) a soluble alkyl or hydride of a metal of Main groups I through III of the periodic table, and optionally catalyst activators, as well as to an apparatus for conducting this process.
As is known, the aforementioned mixed catalysts are capable of polymerizing cyclic olefins having at least one unsubstituted ring double bond, with ring opening (German Published Application No. 1,299,868; and German Unexamined Published Application No. 1,570,940). Such mixed catalysts can effect the disproportionation of open-chain olefins (German Unexamined Published Application No. 1,618,466).
The same catalysts which promote the ring-opening polymerization of cycloolefins are also known to be effective in the olefin metathesis reactions wherein acyclic internal olefins undergo a structural rearrangement, e.g., 2 R.sub.1 --CH=CH--R.sub.2 .fwdarw. R.sub.1 CH=CHR.sub.1, + R.sub.2 CH=CHR.sub.2 as described, inter alia, by Calderon et al., Tetrahedron Letters 1967; 3327, J. Am. Chem. Soc. 90; 4132 (1968) and in C & E News 45 (41); 51 ff. (1969).
It is also known from German Pat. No. 1,929,140 and German Unexamined Published Applications 2,058,183 and 2,016,471 to use these mixed catalysts in producing copolymers from polyolefinically unsaturated hydrocarbon polymers and either cyclic or acyclic olefinically unsaturated hydrocarbon monomers.
Furthermore, it is known from German Unexamined Published Application No. 1,954,092 to produce graft polymers with the aid of these mixed catalysts by reacting cyclopentene with polybutadienes of various microstructures or with butadiene-styrene block polymers.
Finally, German Unexamined Published Application No. 2,058,198 describes reactions using these mixed catalysts of cycloolefins or polyalkenamers with polyolefinically unsaturated hydrocarbon polymers, inter alia also of EPDM rubber with 1,4-polybutadiene.
Such grafting of longer or shorter unsaturated polydiene side chains onto an EPDM rubber backbone is a useful procedure for solving many of the problems caused by the ternary component, i.e., the diene. The presence of a ternary or diene monomer is required in preparing ethylene-propylene copolymers suitable for elastomer use to give curing rates comparable to those of SBR. A variety of such diene monomers are used for this purpose, principally ethylidine norbornene and 1,4-hexadiene. While EPDM elastomers have many desirable properties, the high cost of the ternary monomer has greatly limited their present potential fields of application. The ternary component must meet several practical requirements which can be fulfilled simultaneously only with great difficulties. Principally, the ternary component should not be impracticably expensive, must copolymerize well with ethylene and propylene, and must impart to the terpolymer the ability to rapidly vulcanize to shaped articles having satisfactory properties suitable for the intended end use in practical applications.
In practice, one is forced to compromise the above criteria in selecting the ternary component. For example, inexpensive dicyclopentadiene can be satisfactorily polymerized, but yields terpolymers which are only gradually vulcanized. On the other hand, when using the likewise inexpensive cis,cis-1,5-cyclooctadiene, rapidly vulcanizing terpolymers are produced, but the ternary monomer must be used in large excess due to its poor ability to copolymerize, and the large unreacted proportion must then be recovered and processed from the polymerization system. 5-Ethylidene-norbornene has favorable characteristics with regard to both copolymerization and vulcanization, but is presently available only at a relatively high price.
A substantial advance in the art would be achieved by making it possible to employ an inexpensive, readily polymerizable ternary comonomer, the remaining double bond of which would be easily vulcanizable. A substantial improvement in the previously inadequate covulcanizability of EPDM rubbers with conventional polydiene rubbers would result from a successful grafting of more strongly unsaturated diene rubber chains onto EPDM rubbers. This property plays an important part in such fields of use where the excellent ozone resistance of the EPDM rubbers is exploited in order to protect polydiene rubbers to be joined therewith from disadvantageous atmospheric influences, e.g., in sidewalls for automobile tires and cover strips therefor.
However, the direct joining of shaped objects made of EPDM rubbers with shaped objects made of polydiene rubbers fails due to both the deficient building tack of the former and the mutual physical incompatability of the two compositions. When attempting to improve adhesion by the physical admixture of polydiene rubber with the EPDM mass, the physical incompatibility of the two compositions prevents the formation of genuine homogeneous mixtures. Furthermore, the great difference in the double bond content of the two types of rubber has the effect in conventional sulfur vulcanization, that the EPDM portion of the mixture is normally undervulcanized (i.e., has too low a crosslinking density) if the polydiene proportion of the mixture is in the correct vulcanization condition, whereas the polydiene portion of the mixture is overvulcanized if the EPDM proportion is in the correct vulcanization condition. Such rubber mixtures do not yield the arithmetic mean of the practical-use properties of the individual components after vulcanization, but rather exhibit pronounced minima.
A detailed discussion of these problems is set forth in the literature article by G. Kerrutt, H. Blumel, and H. Weber entitled "Natural and Synthetic Rubber" in Plastics 22: 413-418 (1969). It can be seen therefrom, inter alia, that the mechanical strength of such compositions has a minimally tolerable value only at a ratio of about 75 parts by weight of polydiene rubber to 25 parts by weight of EPDM rubber. However, this proportion of EPDM rubber is too low to still impart the desired high ozone resistance to the mixture.
Accordingly, there is an urgent technical need for an unsaturated polyolefin rubber which is physically compatible with polydiene rubbers and which can be well covulcanized therewith. In such a case, the practical-use properties of the vulcanized mixtures would not show any pronounced minima, but would follow the law of mixtures in the first approximation, i.e., the nearly ideal addition of the properties of several components in a mixture.
During the linking between different types of unsaturated polymers by so-called "cross methathesis" reactions, side reactions occur which strongly reduce the utility of the copolymers so obtained. For example, EPDM rubber tends strongly toward gelling in the presence of the above-defined catalysts; German Unexamined Published Application No. 2,058,198 describes, in Example 2, the reaction product obtained from the cross metathesis of EPDM and 1,4-polybutadiene as a "flabby gel" which must first be rendered soluble again by mechanical breakdown, e.g., grinding or pulverizing. An additional disadvantage is that 1,4-polybutadiene suffers strong degradation in the presence of these catalysts as can be seen from the examples of German Unexamined Published Application No. 2,016,840. Thus, in the final analysis, copolymers having a property spectrum suitable for such practical uses cannot be obtained in accordance with the indicated present state of the art.
These undesired side reactions occur to an increasing extent with lengthening reaction periods, so that they can only be prevented by correspondingly short reaction times, if no further measures are taken. Due to the viscosity of the polymer solutions and the agitating and mixing problems connected therewith, it is difficult to realize such correspondingly brief reaction times, even on an experimental scale.